The invention relates in general to radio frequency filter. In particular it relates to filter structures comprising piezoelectric resonators, typically thin film bulk acoustic wave (BAW) resonators.
The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units. The development leads to increasing requirements on the miniaturization of the components and structures used in the mobile communication means. This development concerns radio frequency (RF) filter structures as well, which despite the increasing miniaturization should be able to withstand considerable power levels, have very steep passband edges, and low losses.
The RF filters used in prior art mobile phones are usually discrete surface acoustic wave (SAW) or ceramic filters. Surface acoustic wave (SAW) resonators typically have a structure similar to that shown in FIG. 1. Surface acoustic resonators utilize surface acoustic vibration modes of a solid surface, in which modes the vibration is confined to the surface of the solid, decaying quickly away from the surface. A SAW resonator typically comprises a piezoelectric layer 100, and two electrodes 122, 124. These electrodes form an Interdigital Transducer (IDT). The shape of the electrodes 122, 124 typically resembles letter E or a comb, and the electrodes are placed so that the fingers of a first electrode are parallel with the fingers of a second electrode and between them. The frequency of a SAW resonator depends most on the distance between the fingers and also on the width of the fingers. Impedance of a SAW resonator depends mostly on the number of fingers and on the length of the fingers. In addition to the IDT, a SAW resonator has typically two reflectors, one on each side of the IDT, for reflecting back the surface acoustic wave induced by the IDT and traversing in a direction normal to the direction of the fingers of the IDT.
Various resonator structures such as filters are produced with SAW resonators. A SAW resonator has the advantage of having a very small size, but unfortunately a SAW resonator cannot withstand high power levels.
It is known to construct thin film bulk acoustic wave resonators on semiconductor wafers, such as silicon (Si) or gallium arsenide (GaAs) wafers. For example, in an article entitled xe2x80x9cAcoustic Bulk Wave Composite Resonatorsxe2x80x9d, Applied Physics Letters, Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is disclosed which comprises a thin film piezoelectric layer of zinc oxide (ZnO) sputtered over a thin membrane of silicon (Si). Further, in an article entitled xe2x80x9cAn Air-Gap Type Piezoelectric Composite Thin Film Resonatorxe2x80x9d, I5 Proc. 39th Annual Symp. Freq. Control, pp. 361-366, 1985, by Hiroaki Satoh, Yasuo Ebata, Hitoshi Suzuki, and Choji Narahara, a bulk acoustic wave resonator having a bridge structure is disclosed.
FIG. 2 shows one example of a bulk acoustic wave resonator having a bridge structure. The structure comprises a membrane 130 deposited on a substrate 200. The resonator further comprises a bottom electrode 110 on the membrane, a piezoelectric layer 100, and a top electrode 120. A gap 210 is created between the membrane and the substrate by etching away some of the substrate from the top side. The gap serves as an acoustic isolator, essentially isolating the vibrating resonator structure from the substrate.
In the following, certain types of BAW resonators are first described. Bulk acoustic wave resonators are typically fabricated on silicon (Si), gallium arsenide (GaAs), glass, or ceramic substrates. One further ceramic substrate type used is alumina. The BAW devices are typically manufactured using various thin film manufacturing techniques, such as for example sputtering, vacuum evaporation or chemical vapor deposition. BAW devices utilize a piezoelectric thin film layer for generating the acoustic bulk waves. The resonance frequencies of typical BAW devices range from 0.5 GHz to 5 GHz, depending on the size and materials of the device. BAW resonators exhibit the typical series and parallel resonances of crystal resonators. The resonance frequencies are determined mainly by the material of the resonator and the dimensions of the layers of the resonator.
A typical BAW resonator consists of three basic elements:
an acoustically active piezoelectric layer,
electrodes on opposite sides of the piezoelectric layer, and
acoustical isolation from the substrate.
The piezoelectric layer may be for example, ZnO, AlN, ZnS or any other piezoelectric material that can be fabricated as a thin film. As a further example, also ferroelectric ceramics can be used as the piezoelectric material. For example, PbTiO3 and Pb(ZrxTi1xe2x88x92x)O3 and other members of the so called lead lanthanum zirconate titanate family can be used.
The material used to form the electrode layers is an electrically conductive material. The electrodes may be comprised of for example any suitable metal, such as tungsten (W), aluminum (Al), copper (Cu), molybdenum (Mo), nickel (Ni), titanium (Ti), niobium (Nb), silver (Ag), gold (Au), and tantalum (Ta). The substrate is typically composed of for example Si, SiO2, GaAs, glass, or ceramic materials.
The acoustical isolation can be produced with for example the following techniques:
with a substrate via-hole,
with a micromechanical bridge structure, or
with an acoustic mirror structure.
In the via-hole and bridge structures, the acoustically reflecting surfaces are the air interfaces below and above the devices. The bridge structure is typically manufactured using a sacrificial layer, which is etched away to produce a free-standing structure. Use of a sacrificial layer makes it possible to use a wide variety of substrate materials, since the substrate does not need to be modified very much, as in the via-hole structure. A bridge structure can also be produced using an etch pit structure, in which case a pit has to be etched in the substrate or the material layer below the BAW resonator in order to produce the free standing bridge structure.
FIG. 3 illustrates one example of various ways of producing a bridge structure. Before the deposition of other layers of the BAW structure, a sacrificial layer 135 is deposited and patterned first. The rest of the BAW structure is deposited and patterned partly on top of the sacrificial layer 135. After the rest of the BAW structure is completed, the sacrificial layer 135 is etched away. FIG. 3 shows also the substrate 200, a membrane layer 130, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. The sacrificial layer can be realized using for example ceramic, metallic or polymeric material.
In the via-hole structure, the resonator is acoustically isolated from the substrate by etching away the substrate from under a major portion of the BAW resonator structure. FIG. 4 shows a via-hole structure of a BAW resonator. FIG. 4 shows the substrate 200, a membrane layer 130, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. A via-hole 211 has been etched through the whole substrate. Due to the etching required, via-hole structures are commonly realized only with Si or GaAs substrates.
A further way to isolate a BAW resonator from the substrate is by using an acoustical mirror structure. The acoustical mirror structure performs the isolation by reflecting the acoustic wave back to the resonator structure. An acoustical mirror typically comprises several layers having a thickness of one quarter wavelength at the center frequency, alternating layers having differing acoustical impedances. The number of layers in an acoustic mirror is typically ranging from three to nine. The ratio of acoustic impedance of two consecutive layers should be large in order to present as low acoustic impedance as possible to the BAW resonator, instead of the relatively high impedance of the substrate material. In the case of a piezoelectric layer that is one quarter of the wavelength thick, the mirror layers are chosen so that as high acoustic impedance as possible is presented to the resonator. This is disclosed in U.S. Pat. No. 5,373,268. The material of the high impedance layers can be for example gold (Au), molybdenum (Mo), or tungsten (W), and the material of the low impedance layers can be for example silicon (Si), polysilicon (poly-Si), silicon dioxide (SiO2), aluminum (Al), or a polymer. Since in structures utilizing an acoustical mirror structure, the resonator is isolated from the substrate and the substrate is not modified very much, a wide variety of materials can be used as a substrate. The polymer layer may be comprised of any polymer material having a low loss characteristic and a low acoustic impedance. Preferably, the polymer material is such that it can withstand temperatures of at least 350xc2x0 C., since relatively high temperatures may be needed during deposition of other layers of the acoustical mirror structure and other structures. The polymer layer may be comprised of, by example, polyimide, cyclotene, a carbon-based material, a silicon-based material or any other suitable material.
FIG. 5 shows an example of a BAW resonator on top of an acoustical mirror structure. FIG. 5 shows the substrate 200, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. The acoustical mirror structure 150 comprises in this example three layers 150a, 150b. Two of the layers 150a are formed of a first material, and the third layer 150b in between the two layers is formed from a second material. The first and second materials have different acoustical impedances as described previously. The order of the materials can be varied. For example, the material with a high acoustical impedance can be in the middle and the material with a low acoustical impedance on both sides of the middle material, or vice versa. The bottom electrode may also be used as one layer of the acoustical mirror.
FIG. 6 shows a further example of a BAW resonator structure. The BAW resonator illustrated in FIG. 6 is a stacked resonator structure having two piezoelectric layers 100. In addition to the bottom 110 and top 120 electrodes, a stacked structure requires a middle electrode 115, which is connected to ground potential. FIG. 6 further shows the membrane layer 130, the substrate 200 and the etch pit 210 isolating the structure from the substrate.
Radio frequency filters, for example, may be constructed using piezoelectric resonators. The resonance frequencies of SAW resonators and of thin film BAW resonators are such that it is advantageous to use them in filters, which are designed to operate at a certain frequency band at a frequency range from about 1 GHz to a few GHz. FIG. 7a presents, as an example, a ladder filter 700 having three similar ladder sections 710, each ladder section 710 consisting of a resonator 701 connected in series and of a second resonator 702 connected in parallel. The series resonance frequency fss of the series resonator 701 and the parallel frequency fpp of the shunt resonator 702 are typically at or close to the center frequency of the ladder filter 700. When a filter is constructed using BAW resonators, the difference in the resonance frequencies of the series resonators 701 and shunt resonators 702 is typically achieved by adding a thin layer of material to the shunt resonators 702. This additional layer increases the resonator thickness and decreases the resonance frequency.
A ladder filter is typically constructed by connecting a certain number of ladder filter sections 710 in series, for achieving the desired characteristics of the filter. Ladder filters typically exhibit fairly good performance in the pass band, but achieving good out-of-band attenuation requires many filter stages, that is ladder sections connected in series. Mobile telephony applications, for example, typically require at least three stages in a ladder filter to achieve desired out-of-band attenuation. FIG. 7b illustrates electric response of one particular ladder filter having the structure specified in FIG. 7a, where the resonators are BAW resonators and the series resonance frequency fss of the series resonator 701 is close to the parallel resonance frequency fpp of the shunt resonator 702. The electric responses in FIG. 7b are calculated. The lower curve corresponds to a ladder filter, where the capacitance (or area) of a shunt resonator is twice that of a series resonator; i.e. the ratio of capacitances of the shunt and series resonators is 2. The upper curve corresponds to a ladder filter, where the capacitances (areas) of the series and shunt resonators are equal, i.e. the ratio is 1.
FIG. 8a presents an example of a lattice filter structure 800 having two lattice sections 810 connected in series; in each lattice section 810 resonators 801a and 801b have a higher resonance frequency than resonators 802a and 802b Lattice filters typically exhibit good out-of-band attenuation far from the pass band, but attenuation near the pass band is not very good. To obtain desired attenuation near pass band, lattice filters typically have a number of lattice sections connected in series. FIG. 8b illustrates electric response of one particular two-stage lattice filter, that is a filter having the structure specified in FIG. 8a. Furthermore, the series and shunt BAW resonators of the lattice filter, whose calculated electric response is illustrated in FIG. 8b, have equal capacitances (areas). The BAW resonators 701, 702 of that ladder filter 700 and BAW resonators 801, 802 of that lattice filter 800, whose electrical responses are presented in FIGS. 7b (upper curve) and 8b, are identical and the parallel resonance frequency fpp of the resonators 702/802 is close to the series resonance frequency fss of the resonators 701/801. FIGS. 7b and 8b clearly demonstrate that the attenuation near the pass band is much better in a ladder filter and the attenuation far from the pass band is much better in a lattice filter. If it is desired to have similar attenuation notches close to the pass band edge (i.e. above and below the passband), as in the case of a ladder filter, then the resonators of a lattice filter, should have an area ratio of less than one (that is, the area of the resonators whose series resonance is close to center frequency should have the larger area), which in turn decreases the out-of-band attenuation far from the passband. This phenomenon is discussed, for example, in patent applications EP1017170 and FI982824. One problem in using ladder filters and lattice filters is that a number of ladder sections or lattice sections are required to achieve good out-of-band attenuation. Such filters having many filter sections have quite a number of piezoelectric resonators. Lattice filters, which have a symmetric structure, furthermore require balanced input and output ports. An antenna typically provides an unbalanced output. Therefore, when a lattice filter is used in current mobile telephony applications, arranging a balanced input requires further components and typically causes further losses.
A further problem with ladder filters is that they have a fairly limited range of relative bandwidths. The maximum bandwidth is limited by the coupling coefficient of the piezoelectric material used in the piezoelectric resonators. The bandwidth of a ladder filter may be decreased by under-tuning BAW resonators, but the response starts to degrade fairly soon, as the standing wave ratio becomes large and insertion loss increases. In an optimal ladder filter, the parallel resonance frequency fpp of a shunt resonator is equal to the series resonance frequency fss of a series resonator. If it is desired to widen the pass band, frequency fpp is decreased and/or frequency fss is increased; this typically, however, causes the dent in the middle of the electric response to deepen. Alternatively, it is possible to decrease the pass band by cross-tuning the series and shunt resonators, i.e. to decrease frequency fss and/or to increase frequency fpp. Also in this case, the electric response typically degrades.
An object of the invention is to present a filter structure having unbalanced input and output ports and enabling good in-band and out-of-band characteristics. A further object of the invention is to present a filter structure having a moderate number of components.
Objects of the invention are achieved with a novel filter section forming either a filter structure or a part of a filter structure.
A filter structure according to the invention comprises a filter section having at least two branches connected in parallel, first branch of said at least two branches comprising a first plurality of piezoelectric resonators connected in series and a second branch of said at least two branches comprising a second plurality of piezoelectric resonators and phase shifting means connected in series, said phase shifting means arranged to provide a phase shift of substantially 180 degrees and piezoelectric resonators belonging to said second plurality having a second resonance frequency, said second resonance frequency being different from the first resonance frequency.
In another aspect of the invention, an arrangement for transmitting and receiving radio frequency signal comprises first amplification means for amplifying a first signal, second amplification means for amplifying a second signal, and a filter structure comprising a first filter branch for filtering the first signal and a second filter branch for filtering the second signal, said first filter branch having a first input conductor and a first output conductor and said second filter branch having a second input conductor and a second output conductor, said first output conductor being connected to said second input conductor, said first input conductor being coupled to an output of the first amplification means and said second output conductor being coupled to an input of the second amplification means, and at least one of said first and second filter branches comprising a filter section having at least two branches connected in parallel, first branch of said at least two branches comprising a first plurality of piezoelectric resonators connected in series, piezoelectric resonators belonging to said first plurality having a first resonance frequency, and a second branch of said at least two branches comprising a second plurality of piezoelectric resonators and phase shifting means connected in series, said phase shifting means arranged to provide a phase shift of substantially 180 degrees and piezoelectric resonators belonging to said second plurality having a second resonance frequency, said second resonance frequency being different from the first resonance frequency.
In a further aspect of the invention, an arrangement for transmitting radio frequency signal comprises a single-ended filter structure providing a port for coupling thereto an antenna, and amplification means connected to said filter structure for amplifying a signal to be transmitted before filtering said signal, and said filter structure comprising a filter section having at least two branches connected in parallel, first branch of said at least two branches comprising a first plurality of piezoelectric resonators connected in series, piezoelectric resonators belonging to said first plurality having a first resonance frequency, and a second branch of said at least two branches comprising a second plurality of piezoelectric resonators and phase shifting means connected in series, said phase shifting means arranged to provide a phase shift of substantially 180 degrees and piezoelectric resonators belonging to said second plurality having a second resonance frequency, said second resonance frequency being different from the first resonance frequency.
In a further aspect of the invention, an arrangement for receiving radio frequency signal comprises a filter structure providing a port for coupling thereto an antenna, and amplification means connected to said filter structure for amplifying a filtered signal, and said filter structure comprising a filter section having at least two branches connected in parallel, first branch of said at least two branches comprising a first plurality of piezoelectric resonators connected in series, piezoelectric resonators belonging to said first plurality having a first resonance frequency, and a second branch of said at least two branches comprising a second plurality of piezoelectric resonators and phase shifting means connected in series, said phase shifting means arranged to provide a phase shift of substantially 180 degrees and piezoelectric resonators belonging to said second plurality having a second resonance frequency, said second resonance frequency being different from the first resonance frequency.
The accompanying dependent claims describe some preferred embodiments of the invention.
A filter section having at least two parallel branches is described here. A first branch of the filter section has a plurality of piezoelectric resonators connected in series. The number of resonators in this plurality is at least one. A second branch of the filter section has a plurality of piezoelectric resonators and phase shifting means providing a phase shift of substantially 180 degrees connected in series. The number of resonators in this second plurality is also at least one. Typically the branches have no further components, so that the first branch consists of the first plurality of piezoelectric resonators connected in series and the second branch consists of the second plurality of piezoelectric resonators and phase shifting means connected in series. The most simple filter section according to this invention thus comprises two piezoelectric resonatorsxe2x80x94one series resonator and one shunt resonatorxe2x80x94and suitable phase shifting means connected in series with the shunt resonator. The piezoelectric resonators may be bulk acoustic wave resonators having, for example, any structure described above. They may alternatively be surface acoustic wave resonators.
The electrical response of a filter section having the specified structure is practically similar to that of a lattice filter section having similar piezoelectric resonators, but the number of resonators is half of the number of resonators in a lattice filter section. Therefore the specified filter section is hereafter in this description called a half-lattice filter section. The number of components in a filter required to produce a desired electric response may thus be decreased significantly using a half-lattice filter section. This is one advantage of the invention. Furthermore, the half-lattice filter section is single ended and it may easily be used, for example, in mobile telephony applications.
It is also possible to combine the half-lattice filter section with, for example, ladder filter section(s). This way it is possible to design and construct single-ended filters, which have a moderate number of components, to meet the needs of various applications. The electric response of such a filter typically has the good out-of-band properties of a lattice filter, steep transitions from pass band to stop band similar to those of ladder filter and the good in-band properties of a ladder and lattice filter. In addition, a wider range of useful relative bandwidths may be available.
Some examples, where a filter structure according to the invention may be used, are radio transmitters, receivers and transceivers (especially mobile telephones). A half-lattice filter section according to the invention may form a part of a duplex filter used in a transceiver; both the receiver and transmitter branch of such a duplex filter may comprise half-lattice filter sections.